High-pressure radical process has been known for long as a process for producing polyethylene having branch structure by polymerization of ethylene. According to this process, a low-density polyethylene having complicated branch structure including short-chain branches and long-chain branches is obtained by homopolymerization of ethylene. However, since the high-pressure radical process utilizes a radical reaction, the branching structure of the resulting ethylene polymer can hardly be controlled, and mechanical strength of the ethylene polymer is not necessarily satisfactory. On the other hand, ethylene-α-olefin copolymers obtained by copolymerization of ethylene and α-olefin using an olefin polymerization catalyst comprising a transition metal, such as Ziegler-Natta catalyst or metallocene catalyst, have a short-chain branching structure of a fixed length originating from α-olefin, and hence are excellent in mechanical strength. In this case, however, α-olefin which is more expensive than ethylene must be used as a starting material.
On the other hand, recently, it has been proposed that a branched polyethylene having only ethyl branch which is a short-branch and is competitive in cost is obtained by polymerization of ethylene using a homogeneous transition metal catalyst comprising a meso-metallocene compound and methylalumoxane (Non-Patent Documents 1 and 2).
[Non-Patent Document 1] Lorella Izzo, Lucia Caporaso, Gerardo Senatore, Leone Oliva, “Branched Polyethylene by Ethylene Homopolymerization with meso-Zirconocene Catalyst”, Macromolecules, U.S.A, American Chemical Society, 1999, Vol. 32, No. 21, p. 6913-6916.
[Non-Patent Document 2] Gianluca Melillo, Lorella Izzo, Roberto Centore, Angela Tuzi, Alexander Z. Voskoboynikov, Leone Oliva, “meso-Me2Si(1-indenyl)2ZrC12/methylalumoxane catalyzed polymerization of the ethylene to ethyl-branched polyethylene”, Journal of Molecular Catalysis A: Chemical), Holland, ELSEVIER, 2005, Vol. 230, p. 29-33.